<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>语句</title>
</head>

<body>
  <script>
    let username
    console.log(undefined == null);
    if (username == null) {
      console.log(username = 'wct');
    }

    i = 1; j = 2;
    k = 2;
    if (i === j) {
      if (i === k)
        console.log('i等于k');
      else
        console.log('i不等于k');
    }
    else {
      console.log('i不等于j');
    }
  </script>
</body>

</html>